Optical sensors provide a convenient means for remote sensing systems, and enable the monitoring of locations large distances from an interrogation location.
FIG. 1 shows a schematic diagram of an optical sensing system comprising four sensing regions X, Y, Z, P. The optical system comprises a main optical fibre 100 through which light propagates. Each sensing region produces an optical phase change in light propagating through the sensor region in response to a change in the natural phenomena being monitored. For example, a hydrophone may detect changes in pressure or accelerometers may detect a change in velocity. Typically such sensors produce a change in physical length of the optical fibre and hence an optical phase change at the output of the sensor, but other mechanisms may be utilised.
In the system of FIG. 1, optical couplers 101a, 101b, 101c, 101d are provided between each sensor region to couple a portion of the light propagating in the main fibre 100 into a branch fibre 102a, 102b, 102c, 102d. The branch fibres are each terminated by a mirror 103a, 103b, 103c, 103d. A mirror 103e terminates the end of the main optical fibre 100.
An interrogation system 104 comprises at least one light source coupled to transmit light into the main optical fibre 100, and at least one optical receiver coupled to receive light returning from that fibre. The interrogation system may be provided in a distributed manner such that elements are located in different locations. For example, it may be desirable position the optical receiver and sampling systems in proximity to the optical sensors (for example on the sea bed) and to transmit the data to a processing location in a convenient place for analysis.
Light reflected by pairs of mirrors either side of each sensor region interferes at the receiver, with the power of the resulting signal being dependent on the optical phase difference between the returning signals. That phase difference is dependent on the optical phase length of the relevant sensor region, and hence will change depending on that phase length. Changes in the detected power can thus be used to detect changes in sensor phase length and hence the physical phenomena to which the sensor responds.
The output of such optical sensors is cyclical with the optical phase length and thus the change between subsequent samples that can be unambiguously resolved is limited to +/−π radians.
FIG. 2 shows an example of a series of 8 samples from an optical sensing system. FIGS. 3a and 3b show these on a phase circle. At sample #0 the phase is assumed to be 0 as the starting point for the measurement. At samples #1-#3 the phase increases in relatively small increments and can be tracked unambiguously. Between sample #3 and #4 the measured phase “wraps” around from +0.95π to 0.3π (shown by the hollow circle in FIG. 2). However, by making the assumption that the phase never changes by more than π between samples, we can unambiguously resolve the actual total phase change and infer that the phase has continued to increase to +1.7π in relation to the starting phase. Sample #5 shows a measured phase of +0.65π, and by making the above assumption the total phase change can be unambiguously resolved to 2.65π.
Sample #6 shows a sampled phase of −0.18π. Applying the assumption that the phase change is less than π would result in the calculated overall phase change decreasing to +1.82π (point 201). However, in fact, the phase may have continued to increase and has actually increased to +3.82π (point 202). There is therefore an error of 2π in the overall phase change, and the direction of change has been calculated incorrectly. The error between samples 5 and 6 can never be recovered because it is only by continuous tracking that the absolute change can be resolved.
Optical sensors thus have a limited range between samples which limits the rate of change of phase in the output signal that can be detected unambiguously to less than π/Ts, where Ts=sample period. The limit that this applies to the rate of change of the sensed phenomena will depend on the characteristics of the sensor, but the rate is limited.
There is therefore a need for an optical sensing system capable of resolving larger rates of change.